Utility power systems deliver power to a number of non-linear loads through switching power supplies. These non-linear loads disadvantageously create significant harmonics upon the utility power buses, resulting in unnecessary losses during power transmission. Power factor correction is used to optimize power transfer with minimum line loss with reduced harmonics. By sensing the instantaneous input current and voltage, the output power can be corrected to maintain a constant supply of power with current and voltage in phase. Active power factor correction in switching converters have been utilized. Power factor correction circuits have been disclosed in U.S. Pat. Nos. 5,301,095, 5,321,600 and 5,045,991.
A boost converter can be operated to draw continuous current with reduced harmonics functioning as line-filtering. The conventional boost converter transfers power to the load without electrical isolation between the utility and the load. The lack of electrical isolation prevents the step-down of the output voltage in a single stage power converter. An isolation transformer is used for electrical isolation. The transformer may be a split transformer having a primary winding and two secondary windings. The primary winding with a center tap is connected to a push pull converter and the secondary windings are connected to the load circuits to provide full-wave rectification and symmetrical operation of the transformer without core saturation using push-pull power switching devices. However, the voltage stresses on push-pull power switching devices are twice the reflected output voltage at either side of the primary windings. Consequently, the push-pull boost converter disadvantageously requires the use of costly switching devices in order to achieve the same conduction losses of the conventional boost converter.
Single-ended modified boost converters are suitable for the power factor correction providing input current shaping and EMI reduction as disclosed in U.S. Pat. No. 5,434,767. A power factor correction (PFC) control circuit provides a constant switching frequency pulse-width-modulated (PWM) signal to a switch driver that drives charging and discharging switching transistors in the boost converter. The PFC control circuit monitors the input current, input voltage and output voltage and varies the PWM signal to perfect power factor correction as well as providing over voltage and current limiting protection. The boost converter provides a step-up or step-down output voltage with electrical isolation using the transformer windings. The boost converter is required to operate at a duty ratio of greater than fifty percent so as to provide sufficient time to reset the transformer core. The benefit of operating the converter at a duty ratio of greater than fifty percent is that the voltage stresses on the switching devices are limited to the reflected output voltage across the primary winding. The transformer turn ratio can be selected to provide step-up or step-down output voltages with desired output voltage regulation in the presence of the restricted duty ratio.
The modified boost converter provides a step-up or step-down output voltages with power factor correction while achieving electrical isolation between the line power source and the output load. This boost converter includes two active switching devices, the charging switch Si and the discharging switch S2, coupled in series with the primary winding. By utilizing parasitic or applied capacitance across the discharging switch S2 connected to magnetizing inductance of the transformer primary winding, resonance occurs within the turn-off interval of the discharging S2 switch, thus facilitating zero-voltage switching of the discharging S2 switch for long term reliability with minimal inductive flyback spikes. The turn-on loss of the conventional charging S1 switch is minimized due to the presence of the leakage inductance of the transformer which allows soft switching when switching the S1 switch by providing a smooth diversion of the input inductor choke current from the primary winding into the charging S1 switch.
While the modified boost converter of U.S. Pat. No. 5,434,767 is suitable for driving single primary winding output transformers with PWM power factor correction with zero voltage switching of the discharging S2 switch, the modified boost converter is not designed to accommodate a plurality of power sources driving a single load. It is now desirable to connect a plurality of power sources to a single load, where the plurality of power sources are used intermittently to provide efficient use of all available power. A multiple source power delivery system may include an AC line power source from a utility, a backup power supply from a battery for when the AC line power fails, and a solar collector power source providing cost effective power collection enabling efficient use of the available power sources. If a plurality of modified boost converters where respectively connected between the plurality of power sources and a plurality of primary windings of an output transformer driving the load, cross coupling of short circuits paths occurs between the primary windings of the transformers causing harmonics and distortion during the operation of the multiple converters defeating efficient power factor correction and efficient power transfer. When a plurality of power sources drive a load, the multiple primary windings would be cross coupled in order to connect the converters to the respective primary windings, but such cross coupled primary windings couple short circuit paths in one boost converter to another boost converter, thereby, disrupting power transfer to the load causing power loss and distortion. These and other disadvantages are solved or reduced using the present invention.